The present invention relates in general to an access unit to a local network transmission medium having carrier sense, multiple access with collision detection (CSMA/CD). The present invention relates, more particularly, to local networks of the ETHERNET type in which the transmission medium is comprised, for the transmission as well as reception of signals, of a pair of telephone wires.
In a general way, a data transmission network is comprised of a number of units, including units generally referred to as "data terminal equipment", also referred to as DTE's, or even terminals or stations for the sake of brevity of terminology. These DTE's communicate between each other via a transmission system. The transmission system, for example, can be a bus architecture and may be comprised of two pairs of telephone wires (one for sending, the other for receiving) or a coaxial cable.
Local networks (also referred to as local area networks, LAN's) are transmission networks generally limited to an area of somewhat limited circumference (real estate, plant, campus, hospital) where the distances between stations is from several meters or tens of meters to several kilometers. The transmission of data is done at rates on the order of 100 kbits/sec. to several tens of Mbits/second.
Because the stations of a local network with bus architecture share a single transmission system (usually referred to as a transmission line), it is important that a single station be sending at any one time on the same line. To resolve this problem, the stations or terminals are generally equipped with decision elements that do not authorize the sending of messages unless the station is free. To do this, according to one preferred method, each terminal monitors continuously the signals present on the line and if a message is present on it, no other message is authorized to be sent except by one of the stations other than the sending station. Thus the access methods designed for local bus networks aim to limit access conflicts due to the simultaneous sending by several stations. This access method is referred to as carrier sense, multiple access (CSMA) with collision detection.
However, in spite of these precautions, it can happen that a station does not detect the presence of a message on the transmission line at the moment when it is initiating the sending of a message itself, although another station is in the process of sending. In this type of network, this situation called "collision" is impossible to prevent totally, notably because of the delay in propagation of signals on the line.
In other words, for a specified station, a collision is defined as the simultaneous presence of its own send signal and a signal sent by another station (which, in itself, is a receive signal at the specified station).
To remedy this difficulty, generally a device called a collision detector is used which interrupts the sending of a message in progress when this situation has been detected. The detection of a collision causes the cancellation of the transmission when a new send attempt is made, the send not taking place unless the transmission line is free again. This access method is known as carrier sense, multiple access with collision detection (CSMA/CD).
The local networks of the CSMA/CD type have been standardized by the IEEE (Institute of Electrical and Electronic Engineers) in the form of a standard referred to as 802.3, (summarized by the International Standards Organization (I.S.O.) in the form of standard I.S.O. 8 802.3), of which the essential provisions are the following:
a DTE when it is a sender of data is either silent, or in the process of sending data, PA1 a DTE, when it is silent, may, in the role of receiver, either detect a silence, or receive the data coming from one or several other DTE's. If the data received arrive simultaneously from several DTE's, the value of this data is without significance, which is the case when a collision is present. PA1 a DTE that is in the process of sending, may, in the role of receiver, either detect that its sending is proceeding normally (without collision with the data sent by one or several other DTE's), or that a collision is taking place. PA1 the standardized transmission medium is a coaxial cable with a characteristic impedance of 50 ohms equipped at each end with a 50 ohm termination. Each terminal is connected to the coaxial cable by the medium access unit, commonly referred to as a transceiver. PA1 The standardized transmission medium is composed of two telephone wires, one for sending, the other for receiving. PA1 a sending current generator which receives the signals sent by the station connected to the transceiver and which delivers a send signal in the form of a polarized current (with a non-zero average value), PA1 a generator of receive signals receiving the signals transmitted by the medium arriving from another station than the connected station in the form of a polarized signal (with a non zero average value), PA1 a circuit to analyze the signal received by the connected station, receiving on the one hand the signal sent by it and on the other hand any signal sent by another station, this analysis circuit sending a signal to the connected station indicating if it alone is sending or if there is a collision. The signal sent by the station, which is received by the above mentioned analysis circuit, proceeds from the coaxial cable, and, in fact, any current sent by a station creates a voltage in the terminator fitting which is seen by all the stations via their transceiver. This analysis circuit is in fact a circuit to detect collisions. In practice, it is composed of a threshold circuit permitting determination of whether the average value of the voltage of the signal present on the coaxial cable is greater than a specified value equal to the standard average value of the voltage of the send signal. PA1 the transmission medium does not pass on to the transceiver connected to the sending station the send signal of the latter, the station then ignores whether its own send signal received by the transceiver has been sent by same to the other stations via the transmission medium. PA1 when the station is sending, its own send signal is not being sent back to it by the transmission medium because of the independence of the two pairs of telephone wires one to the other, and it is impossible for the transceiver to detect every collision. This may result in the possibility for each of the stations of the same elementary segment of the network to send at the same time as the others, since each of them is incapable of detecting a collision. PA1 permit each station that is sending to know at every moment that its send signal received by the transceiver has been properly sent by same on the transmission medium, PA1 permit each station that is sending to know if there is a collision on the network and as a consequence, according to the CSMA/CD procedure, to interrupt its send and to repeat it several seconds later, according to the standardized procedure IEEE 802.3. PA1 a send current generator receiving the signals sent by the connected station and delivering a send signal in the form of a polarized current with a non-zero average value, PA1 a receive signal generator receiving the signals arriving from another station sent by the medium in the form of a polarized voltage with a non-zero average value. PA1 a collision detector receiving on the one hand the signal sent by the station and on the other hand every signal sent by another station and sending to the connected station a signal indicating if it is the only one sending or if there is a collision, when it receives a signal with an average voltage exceeding, in absolute value, a predetermined value, PA1 characterized by the fact that the transmission medium is composed of two pairs of telephone wires, one for sending and the other for receiving, it contains a collision simulation circuit simulating the physical superposition of send and receive signals receiving on the one hand a first send voltage obtained from the send current and on the other hand the receive signal sent by another station via the pair of receive telephone wires and re transmitting on the collision detection circuit either the receive signal present on the transmission medium when the connected station is not sending, or even a collision simulation signal obtained from the send voltage and whose non-zero average value is such that the collision detector signals a collision to the connected station when there is the simultaneous presence of a send signal by the associated station and a receive signal. PA1 a circuit detecting every send from the connected station, receiving the first send voltage, sending a first signal of validation indicating that the connected station is sending, PA1 a detector circuit for receive signals, receiving the reception signal transmitted by the receiving telephone pair and sending a second validation signal indicating the presence of a reception signal on the reception pair, PA1 an adaptation circuit for the send voltage receiving on the one hand the first send voltage and on the other hand the second signal validation and supplying either the second send voltage when the connected station is the only one sending, or the collision simulation signal, PA1 a multiplexer receiving the first and the second validation signal, the receive signal, and the second send voltage or the collision simulation signal, and transmitting either the receive signal when the connected station is not sending at the moment, or the second send voltage when the connected station is the only one sending or the collision simulation signal when there is a collision
Likewise, the standard defines the format of the frames. A frame has a given structure: it comprises a start of frame delimiter, an end of frame delimiter, synchronizing signals for obtaining a clock signal, the address of the station to which the message is directed, the address of the sending station, the length of the data, the useful data, etc. In other words, the frame is the elementary block of information sent by any station whatsoever, which goes over the transmission line.
In addition, standard 802.3 defines the protocol governing the dialog between the stations. This defines the rules of access to different stations and thus constitutes a system which schedules the communication between stations without prioritizing it.
In current actual practice, there exist two local networks of the CSMA/CD type, namely the local network of the ETHERNET type (ETHERNET is a trademark registered Feb. 6, 1981 by XEROX CORPORATION) and the local network of the STARLAN type, the rate of data transmission for the former being 10 Mbits/second while the rate for the second is 1 Mbit/second.
The physical configuration of a local network of the ETHERNET type is as follows:
The basic network or elementary segment is composed of a segment of cable along the length of which are distributed access unit connections.
In a very general manner, a local area network is formed by an assembly of several basic networks among which repeaters are placed. These repeaters re-send on a cable the signals that they receive, completely regenerating their initial quality. The functioning of a network of several segments is assured if the network is composed of a principal segment and secondary segments, each secondary segment being connected to the principal segment either by a repeater or by a pair of repeaters connected to each other.
The physical configuration of a local network of the STARLAN type is as follows:
The basic network is comprised of an assembly of terminals, each connected by a pair of telephone wires to a common central hub. This central hub plays the same role as the repeaters in the local network of the ETHERNET type, i.e. re-sends on each of the pairs of telephone wires the signals arriving from any one of the stations, and completely regenerates their initial quality. Several basic networks each containing a central hub may be connected among themselves by other central hubs and thus constitute the properly-called local network. Detection of collisions is done on the level of each central hub.
One of the consequences of current technological progress is that the pairs of telephone wires may in the future transport information with rates on the order of 10 Mbits/second, and indeed more, although this has not been the case for several years. It follows that one of the current tendencies in the development of local area networks of the ETHERNET type consists of using two pairs of telephone lines in place of a coaxial cable, at least for the base networks, as the transmission means. In fact, the telephone pair is much less expensive, for one thing, and for another, its pre-wiring on the property is simpler and less expensive than the pre-wiring of a coaxial cable. For this reason, central hubs that play the same role as those used in the STARLAN network are used, at least for the base networks.
Attempts have been made to use two pairs of telephone wires as the transmission medium in ETHERNET local area networks, the majority of the constituent elements of an ETHERNET local area network using a coaxial cable as the transmission medium.
This is notably the case for the integrated circuit constituting the transceiver used in the ETHERNET local area network with coaxial cable. This type of transceiver is manufactured, for example, by "National Semiconductor" Company (NS) under the commercial designation DP 8392.
The essential component parameters of such a transceiver are the following:
If the coaxial cable is replaced, in the role of transmission medium, by a pair of telephone wires of which one is physically independent of the other, the following problem appears:
A first solution includes equipping the central hubs with collision detectors as is the case in the local area networks of the STARLAN type. This is not the solution that was used for the central hubs of the ETHERNET networks with twisted pair.